A silica glass is used for a lens, a prism and a photomask of a photolithography instrument in manufacturing of a large-scale integrated circuit (LSI), for a TFT substrate used for a display, for a tube of a lamp, for a window material, for a reflection plate, for a cleaning container in a semiconductor industry, for a container for melting of a silicon semiconductor, and so forth. However, an expensive compound such as silicon tetrachloride must be used as a raw material for these silica glasses; on top of that, melting temperature and processing temperature of a silica glass is extraordinary high, as high as about 2000° C., thereby leading to a high energy consumption and a high cost. Accordingly, from the past, various methods for producing a silica glass have been proposed.
For example, in Patent Document 1, a method (sol-gel method) in which a silicon alkoxide is hydrolyzed to a silica sol, which is then gelated to a wet gel, then to a dry gel by drying, and finally to a transparent silica glass body by heating at high temperature is disclosed. In Patent Document 2, a method in which a transparent silica glass is obtained by a sol-gel method from a silica sol mixture solution formed of tetramethoxy silane or tetraethoxy silane and a silica sol solution containing silica fine particles is disclosed. In Patent Document 3, a method for producing a transparent silica glass by using a silicon alkoxide and silica glass fine particles as its main raw materials, wherein a heating process at a temperature range of 200 to 1300° C. is carried out under an oxygen gas-containing atmosphere, a further heating process to 1700° C. or higher is carried out under a hydrogen gas-containing atmosphere, and a heating process between the foregoing two heating processes is carried out under a reduced pressure atmosphere, is disclosed. In these conventional sol-gel methods, however, the produced silica glass has problems not only in an initial dimensional precision of the produced silica glass and in a heat resistance during its use thereafter at high temperature but also in its production cost which is not so cheap.
In Patent Document 4, a method (slip casting method), wherein at least two different kinds of silica glass particles, for example, silica glass fine particles and silica glass granules are mixed to obtain a water-containing suspension solution, which is then press molded and sintered at high temperature to obtain a silica-containing composite body, is disclosed. In Patent Document 5, a method, wherein a mixed solution (slurry) containing silica glass particles having the size of 100 μm or less and silica glass granules having the size of 100 μm or more is prepared, then the slurry is cast into a molding frame, dried, and then sintered to obtain an opaque silica glass composite material, is disclosed. In these conventional slip casting methods, however, shrinkage of a molded article in a drying process and a sintering process is so significant that a thick silica glass article with a high dimensional precision could not be obtained.
Accordingly, there are problems in each method for producing a silica glass article as mentioned above. Therefore, as a method for producing a silica crucible for manufacturing of a single crystal silicon used for LSI (for a device), such production methods as those disclosed in Patent Document 6 and Patent Document 7 are being used still today. In these methods, after a powdered, ultra-highly purified natural quartz or a powdered synthetic cristobalite is fed into a rotating frame made of carbon and then molded, carbon electrodes are inserted from the top and then electrically charged thereby causing arc discharge to raise the atmospheric temperature to a temperature range for melting of the powdered quartz (temperature is estimated in the range from about 1800 to about 2100° C.) so that the powdered raw quartz may be melted and sintered.
In the methods such as those mentioned above, however, there has been a problem of a high cost because a powdered raw material quartz with high purity is used. In addition, because various kinds of impure gases are dissolved in a produced silica crucible, the gases are released and then incorporated into a silicon single crystal as gaseous bubbles thereby causing such problems as defects called a void and a pinhole when it is used as a silica crucible for growing of a silicon single crystal; and thus this has been causing problems in production cost as well as quality of the silicon crystal. In addition, there has been a big problem in durability of the silica crucible because of low etching resistance to silicon melt at the time of pulling up of a single crystal silicon.
A method to improve the etching resistance to the silicon melt in a silica crucible for pulling up of the single crystal is shown in Patent Document 8. In Patent Document 8, an effect of applying a crystallization accelerator on an inner surface of a silica glass crucible is shown. As the crystallization accelerator, Mg, Sr, Ca, and Ba, which are alkaline earth metal elements belonging to the 2a group, and Al, which is the element belonging to the 3b group, are shown. However, a silica glass crucible as shown in Patent Document 8 was not the one having a transparent silica glass layer completely free from gaseous bubbles at an inner surface part of the crucible, but the one containing micro gaseous bubbles and inhomogeneously undissolved particles of various doped elements. Accordingly, there have been problems frequently that a pulled-up silicon single crystal contains silica fine particles as foreign substances and has defects such as voids and pinholes.
Patent Document 9 shows a technology to reduce gaseous bubbles in a silica glass of an inner surface part of a silica crucible for pulling-up of a single crystal so that bubble expansion in the silica crucible in use may be suppressed. In Patent Document 9, it is shown that expansion of the gaseous bubbles in an inner surface of the silica crucible, generated during pulling-up of a single crystal at high temperature and under reduced pressure, can be suppressed by incorporating hydrogen molecules into a powdered raw material for the silica crucible with the concentration of 5×1017 to 3×1019 molecules/cm3. In this method, however, there still has been a problem in durability of the silica crucible because of low etching resistance to silicon melt at the time of pulling up of a single crystal silicon.